A theoretical and spectroscopic study of co-amorphous naproxen and indomethacin

Development of a screening method for co-amorphous formulations of drugs and amino acids

Using amino acids (AA) as low molecular weight excipients in the preparation of co-amorphous blends with the aim to stabilize the drug in the amorphous form have been discussed in a range of studies. However, there is currently no theoretical consensus behind which AA would be a suitable co-former for a given drug. In this work, a fast screening process to assess the co-former feasibility in co-amorphous drug-AA blends has been developed on the basis of the amorphization kinetics upon oscillatory ball milling. For this purpose, six model drugs were combined with 20 different AAs and co-milled at an equimolar ratio for different times (1, 5, 15, 30 and 60min). The degree of amorphization was then studied for the different time points by determination of the area under the curve of the diffraction peaks in X-ray powder diffraction measurements. The results of this study suggest that the choice of AA as co-formers for the formation of the co-amorphous blend could be significantly inferred after 15min of milling, since a crystallinity decrease higher than 90% after 15min resulted in successful co-amorphization in approximately 90% of the mixtures after 60min of milling. The results furthermore suggested that non-polar AAs, such as tryptophan, phenylalanine, leucine, isoleucine, methionine, valine and proline, are a good first choice in the selection of a co-former for a given drug in a co-amorphous formulation. Basic AAs appear suitable for amorphous salt formation in the case of acidic drugs. Acidic AAs however, were shown to be generally poor co-formers for co-amorphous systems.

Influence of variation in molar ratio on co-amorphous drug-amino acid systems

Molecular interactions were investigated within four different co-amorphous drug-amino acid systems, namely indomethacin-tryptophan (Ind-Trp), furosemide-tryptophan (Fur-Trp), indomethacin-arginine (Ind-Arg) and furosemide-arginine (Fur-Arg). The co-amorphous systems were prepared by ball milling for 90min at different molar ratios and analyzed by XRPD and DSC. Interactions within the co-amorphous samples were evaluated based on the deviation between the actual glass transition temperature (Tg) and the theoretical Tg calculated by the Gordon-Taylor equation. The strongest interactions were observed in the 50mol% drug (1:1M ratio) mixtures, with the exception of co-amorphous Ind-Arg where the interactions within the 40mol% drug samples appear equally strong. A particularly large deviation between the theoretical and actual Tgs was observed within co-amorphous Ind-Arg and Fur-Arg systems. Further analysis of these co-amorphous systems by (13)C solid-state NMR (ssNMR) and FTIR confirmed that Ind and Fur formed a co-amorphous salt together with Arg. A modified approach of using the Gordon-Taylor equation was applied, using the equimolar co-amorphous mixture as one component, to describe the evolution of the Tgs with varying molar ratio between the drug and the amino acid. The actual Tgs for co-amorphous Ind-Trp, Fur-Trp and Fur-Arg were correctly described by this equation, confirming the assumption that the excess component was amorphous forming a homogeneous single component within the co-amorphous mixture without additional interactions. The modified equation described the Tgs of the co-amorphous Ind-Arg with excess Arg less well indicating possible further interactions; however, the FTIR and ssNMR data did not support the presence of additional intermolecular drug-amino acid interactions.

Recent developments in the Raman and infrared investigations of amorphous pharmaceuticals and protein formulations: A review

The success rate for drug discovery and the development of innovative therapeutic strategies are intimately related to the physical properties of the solid-state condensed matter, which have direct influence on the bioavailability of Active Pharmaceutical Ingredients. In order to transform a new molecule in efficient drug, the material is brought into an amorphous state using various manufacturing processes including freeze drying, spray drying, hot melt extrusion and loading in different delivery devices. The infrared and Raman spectroscopic analyses used for exploring disordered and amorphous states, for the monitoring of the drug physical stability in drug delivery systems are described in this review.

Recent advances in co-amorphous drug formulations

Co-amorphous drug delivery systems have recently gained considerable interest in the pharmaceutical field because of their potential to improve oral bioavailability of poorly water-soluble drugs through drug dissolution enhancement as a result of the amorphous nature of the material. A co-amorphous system is characterized by the use of only low molecular weight components that are mixed into a homogeneous single-phase co-amorphous blend. The use of only low molecular weight co-formers makes this approach very attractive, as the amount of amorphous stabilizer can be significantly reduced compared with other amorphous stabilization techniques. Because of this, several research groups started to investigate the co-amorphous formulation approach, resulting in an increasing amount of scientific publications over the last few years. This study provides an overview of the co-amorphous field and its recent findings. In particular, we investigate co-amorphous formulations from the viewpoint of solid dispersions, describe their formation and mechanism of stabilization, study their impact on dissolution and in vivo performance and briefly outline the future potentials.

Properties of the Sodium Naproxen-Lactose-Tetrahydrate Co-Crystal upon Processing and Storage

Co-crystals and co-amorphous systems are two strategies to improve the physical properties of an active pharmaceutical ingredient and, thus, have recently gained considerable interest both in academia and the pharmaceutical industry. In this study, the behavior of the recently identified sodium naproxen-lactose-tetrahydrate co-crystal and the co-amorphous mixture of sodium, naproxen, and lactose was investigated. The structure of the co-crystal is described using single-crystal X-ray diffraction. The structural analysis revealed a monoclinic lattice, space group P2_1, with the asymmetric unit containing one molecule of lactose, one of naproxen, sodium, and four water molecules. Upon heating, it was observed that the co-crystal transforms into a co-amorphous system due to the loss of its crystalline bound water. Dehydration and co-amorphization were studied using synchrotron X-ray radiation and thermogravimetric analysis (TGA). Subsequently, different processing techniques (ball milling, spray drying, and dehydration) were used to prepare the co-amorphous mixture of sodium, naproxen, and lactose. X-ray powder diffraction (XRPD) revealed the amorphous nature of the mixtures after preparation. Differential scanning calorimetry (DSC) analysis showed that the blends were single-phase co-amorphous systems as indicated by a single glass transition temperature. The samples were subsequently tested for physical stability under dry (silica gel at 25 and 40 °C) and humid conditions (25 °C/75% RH). The co-amorphous samples stored at 25 °C/75% RH quickly recrystallized into the co-crystalline state. On the other hand, the samples stored under dry conditions remained physically stable after five months of storage, except the ball milled sample stored at 40 °C which showed signs of recrystallization. Under these dry conditions, however, the ball-milled co-amorphous blend crystallized into the individual crystalline components.

Looking for the interactions between omeprazole and amoxicillin in a disordered phase. An experimental and theoretical study

In this paper, co-grinding mixtures of omeprazole-amoxicillin trihydrate (CGM samples) and omeprazole-anhydrous amoxicillin (CGMa samples) at 3:7, 1:1 and 7:3 molar ratios, respectively, were studied with the aim of obtaining a co-amorphous system and determining the potential intermolecular interactions. These systems were fully characterized by differential scanning calorimetry (DSC), FT-infrared spectroscopy (FTIR), X-ray powder diffraction (PXRD), scanning electron microscopy (SEM) and solid state Nuclear Magnetic Resonance (ssNMR). The co-grinding process was not useful to get a co-amorphous system but it led to obtaining the 1:1 CGMa disordered phase. Moreover, in this system both FTIR and ssNMR analysis strongly suggest intermolecular interactions between the sulfoxide group of omeprazole and the primary amine of amoxicillin anhydrous. The solubility measurements were performed in simulated gastric fluid (SGF) to prove the effect of the co-grinding process. Complementarily, we carried out density functional theory calculations (DFT) followed by quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses in order to shed some light on the principles that guide the possible formation of heterodimers at the molecular level, which are supported by spectroscopic experimental findings.

A Strategy for Co-former Selection to Design Stable Co-amorphous Formations Based on Physicochemical Properties of Non-steroidal Inflammatory Drugs

PURPOSE

This study aimed to investigate the physicochemical factors contributing to stable co-amorphous formations and to design a co-former selection strategy.

METHODS

Non-steroidal inflammatory drugs were used as main components and/or co-formers. Physical mixtures of the materials were melted. Co-amorphization was characterized by the inhibition effect of the co-former on crystallization of the main component from the undercooled melt. The contribution of physicochemical factors to the co-amorphous formation was analyzed by multivariate analysis. Co-amorphous samples prepared by melting were subjected to thermal and spectroscopic analyses and the isothermal crystallization test.

RESULTS

Naproxen (NAP) was employed as the main component having a rapid crystallization tendency. Some materials used as the co-former inhibited the crystallization of amorphous NAP; decreasing melting temperatures of the components was an indicator of co-amorphization. The contribution of some physicochemical features (e.g., crystallization tendency, glass transition temperature (Tg)/melting temperature and molecular flexibility) of the co-formers to a co-amorphous formation was suggested by multivariate analysis. Deviation of the glass transition temperature from the theoretical value and changes in the infrared spectra of the co-amorphous samples were correlated with intermolecular interaction. The crystallization behaviors of the co-amorphous samples depended on their Tg.

CONCLUSIONS

The results showed a relationship between stable co-amorphous formation and the physicochemical features of the components, which should inform efficient co-former selection to design stable co-amorphous formations.

Application of Low- and Mid-Frequency Raman Spectroscopy to Characterize the Amorphous-Crystalline Transformation of Indomethacin

Raman spectroscopy using the mid-frequency (1800-1500 cm(-1)) and low-frequency (200-8 cm(-1)) spectral regions is used to study the transformation of amorphous indomethacin (IND) to the γ-crystalline form. The low-frequency spectral region provides access to collective vibrations of molecules in the crystalline and amorphous state, while the mid-frequency spectral region provides access to the molecular vibrations that are sensitive to the local functional group environment. Both spectral regions provide distinct Raman bands for the amorphous and crystalline forms of IND. The more intense low-frequency Raman bands provide greater sensitivity for detecting the onset of crystallization in an amorphous matrix. Subtle differences in the behavior of the initial crystalline process of IND are observed between the low-frequency and mid-frequency Raman bands. These observations suggest that different responses for mid- and low-frequency Raman bands occur for the microcrystalline domains present during the initial crystallization process. The suitability of low-frequency Raman spectroscopy to monitor IND in a suspension was demonstrated. This suggests that the technique will be a valuable tool for at-line and on-line monitoring of active pharmaceutical ingredient crystallization.

Multivariate Quantification of the Solid State Phase Composition of Co-Amorphous Naproxen-Indomethacin

To benefit from the optimized dissolution properties of active pharmaceutical ingredients in their amorphous forms, co-amorphisation as a viable tool to stabilize these amorphous phases is of both academic and industrial interest. Reports dealing with the physical stability and recrystallization behavior of co-amorphous systems are however limited to qualitative evaluations based on the corresponding X-ray powder diffractograms. Therefore, the objective of the study was to develop a quantification model based on X-ray powder diffractometry (XRPD), followed by a multivariate partial least squares regression approach that enables the simultaneous determination of up to four solid state fractions: crystalline naproxen, γ-indomethacin, α-indomethacin as well as co-amorphous naproxen-indomethacin. For this purpose, a calibration set that covers the whole range of possible combinations of the four components was prepared and analyzed by XRPD. In order to test the model performances, leave-one-out cross validation was performed and revealed root mean square errors of validation between 3.11% and 3.45% for the crystalline molar fractions and 5.57% for the co-amorphous molar fraction. In summary, even four solid state phases, involving one co-amorphous phase, can be quantified with this XRPD data-based approach.

Toward a Better Understanding of the Physical Stability of Amorphous Anti-Inflammatory Agents: The Roles of Molecular Mobility and Molecular Interaction Patterns

The aim of this article is to examine the crystallization tendencies of three chemically related amorphous anti-inflammatory agents, etoricoxib, celecoxib, and rofecoxib. Since the molecular mobility is considered as one of the factors affecting the crystallization behavior of a given material, broadband dielectric spectroscopy was used to gain insight into the molecular dynamics of the selected active pharmaceutical ingredients. Interestingly, our experiments did not reveal any significant differences in their relaxation behavior either in the supercooled liquid or in the glassy state. Hence, as a possible explanation for the enhanced physical stability of etoricoxib, its ability to undergo a tautomerization reaction was recognized. The occurrence of intramolecular proton transfer in the disordered etoricoxib was proven experimentally by time-dependent dielectric and infrared (IR) measurements. Additionally, IR spectroscopy combined with density functional theory calculations pointed out that in the etoricoxib drug, being in fact a binary mixture of tautomers, the individual isomers may interact with each other through a hydrogen bonding network. A possible explanation of this issue was achieved by performing dielectric experiments at elevated pressure. Since compression results in etoricoxib recrystallization, the possible influence of pressure on the observed stabilization effect is also carefully discussed.

Thermodynamics of Highly Supersaturated Aqueous Solutions of Poorly Water-Soluble Drugs-Impact of a Second Drug on the Solution Phase Behavior and Implications for Combination Products

There is increasing interest in formulating combination products that contain two or more drugs. Furthermore, it is also common for different drug products to be taken simultaneously. This raises the possibility of interactions between different drugs that may impact formulation performance. For poorly water-soluble compounds, the supersaturation behavior may be a critical factor in determining the extent of oral absorption. The goal of the current study was to evaluate the maximum achievable supersaturation for several poorly water-soluble compounds alone, and in combination. Model compounds included ritonavir, lopinavir, paclitaxel, felodipine, and diclofenac. The "amorphous solubility" for the pure drugs was determined using different techniques and the change in this solubility was then measured in the presence of differing amounts of a second drug. The results showed that "amorphous solubility" of each component in aqueous solution is substantially decreased by the second component, as long as the two drugs are miscible in the amorphous state. A simple thermodynamic model could be used to predict the changes in solubility as a function of composition. This information is of great value when developing co-amorphous or other supersaturating formulations and should contribute to a broader understanding of drug-drug physicochemical interactions in in vitro assays as well as in the gastrointestinal tract.

Anomalous role change of tertiary amino and ester groups as hydrogen acceptors in eudragit E based solid dispersion depending on the concentration of naproxen

Eudragit E (EGE) is a basic polymer incorporating tertiary amino and ester groups. The role of the functional groups of EGE in the formation of solid dispersion (SD) with Naproxen (NAP) was investigated. The glass transition temperature (Tg) of EGE decreased with the plasticizing effect of NAP up to 20% weight ratio. Addition of NAP at over 30% induced a rise in Tg, with the maximum value being reached at 60% NAP. Further addition of NAP led to a rapid drop of the Tg. A dramatic difference of physical stability between the SDs including 60 and 70% NAP was confirmed. The SD including 70% NAP rapidly crystallized at 40 °C with 75% relative humidity, while the amorphous state could be maintained over 6 months in the SD with 60% NAP. The infrared and (13)C solid state-NMR spectra of the SDs suggested a formation of ionic interaction between the carboxylic acid of NAP and the amino group of EGE. The SD with 20% NAP raised the (13)C spin-lattice relaxation (T1) of the amino group, but it decreased with over 30% NAP. The change in the (13)C-T1 disappeared with 70% NAP. The (13)C-T1 of the ester group rose depending on the amount of NAP. From these findings, we concluded that the role as hydrogen acceptor shifted from the amine to the ester group with an increase in amount of NAP. Furthermore, the amino group of EGE did not contribute to the interaction at over 70% NAP. These phenomena could be strongly correlated with Tg and stability.

Stabilization of amorphous paracetamol based systems using traditional and novel strategies

There is a special interest in having pharmaceutical active ingredients in the amorphous state due to their increased solubility and therefore, higher bioavailability. Nevertheless, not all of them present stable amorphous phases. In particular, paracetamol is an active ingredient widely known for its instability when prepared in the amorphous state. In the present work thermally stable amorphous binary paracetamol based systems were obtained showing stability on a wide range of temperatures: below its glass transition temperature (Tg) as amorphous solids in the glassy state and above their glass transition temperature, where these materials exist as stable supercooled liquids. To achieve stabilization of the binary paracetamol based system several strategies were applied and optimized, being the selection of the container material a key and novel approach to control the mechanical stress during cooling, eliminating cracks which act as nucleation centers leading to crystallization.

Improving co-amorphous drug formulations by the addition of the highly water soluble amino Acid, proline

Co-amorphous drug amino acid mixtures were previously shown to be a promising approach to create physically stable amorphous systems with the improved dissolution properties of poorly water-soluble drugs. The aim of this work was to expand the co-amorphous drug amino acid mixture approach by combining the model drug, naproxen (NAP), with an amino acid to physically stabilize the co-amorphous system (tryptophan, TRP, or arginine, ARG) and a second highly soluble amino acid (proline, PRO) for an additional improvement of the dissolution rate. Co-amorphous drug-amino acid blends were prepared by ball milling and investigated for solid state characteristics, stability and the dissolution rate enhancement of NAP. All co-amorphous mixtures were stable at room temperature and 40 °C for a minimum of 84 days. PRO acted as a stabilizer for the co-amorphous system, including NAP–TRP, through enhancing the molecular interactions in the form of hydrogen bonds between all three components in the mixture. A salt formation between the acidic drug, NAP, and the basic amino acid, ARG, was found in co-amorphous NAP–ARG. In comparison to crystalline NAP, binary NAP–TRP and NAP–ARG, it could be shown that the highly soluble amino acid, PRO, improved the dissolution rate of NAP from the ternary co-amorphous systems in combination with either TRP or ARG. In conclusion, both the solubility of the amino acid and potential interactions between the molecules are critical parameters to consider in the development of co-amorphous formulations.

Amino acids as co-amorphous excipients for simvastatin and glibenclamide: physical properties and stability

Co-amorphous drug mixtures with low-molecular-weight excipients have recently been shown to be a promising approach for stabilization of amorphous drugs and thus to be an alternative to the traditional amorphous solid dispersion approach using polymers. However, the previous studies are limited to a few drugs and amino acids. To facilitate the rational selection of amino acids, the practical importance of the amino acid coming from the biological target site of the drug (and associated intermolecular interactions) needs to be established. In the present study, the formation of co-amorphous systems using cryomilling and combinations of two poorly water-soluble drugs (simvastatin and glibenclamide) with the amino acids aspartic acid, lysine, serine, and threonine was investigated. Solid-state characterization with X-ray powder diffraction, differential scanning calorimetry, and Fourier-transform infrared spectroscopy revealed that the 1:1 molar combinations simvastatin-lysine, glibenclamide-serine, and glibenclamide-threonine and the 1:1:1 molar combination glibenclamide-serine-threonine formed co-amorphous mixtures. These were homogeneous single-phase blends with weak intermolecular interactions in the mixtures. Interestingly, a favorable effect by the excipients on the tautomerism of amorphous glibenclamide in the co-amorphous blends was seen, as the formation of the thermodynamically less stable imidic acid tautomer of glibenclamide was suppressed compared to that of the pure amorphous drug. Furthermore, the co-amorphous mixtures provided a physical stability advantage over the amorphous drugs alone.

Amino acids as co-amorphous stabilizers for poorly water soluble drugs--Part 1: preparation, stability and dissolution enhancement

Poor aqueous solubility of an active pharmaceutical ingredient (API) is one of the most pressing problems in pharmaceutical research and development because up to 90% of new API candidates under development are poorly water soluble. These drugs usually have a low and variable oral bioavailability, and therefore an unsatisfactory therapeutic effect. One of the most promising approaches to increase dissolution rate and solubility of these drugs is the conversion of a crystalline form of the drug into its respective amorphous form, usually by incorporation into hydrophilic polymers, forming glass solutions. However, this strategy only led to a small number of marketed products usually because of inadequate physical stability of the drug (crystallization). In this study, we investigated a fundamentally different approach to stabilize the amorphous form of drugs, namely the use of amino acids as small molecular weight excipients that form specific molecular interactions with the drug resulting in co-amorphous forms. The two poorly water soluble drugs carbamazepine and indomethacin were combined with amino acids from the binding sites of the biological receptors of these drugs. Mixtures of drug and the amino acids arginine, phenylalanine, tryptophan and tyrosine were prepared by vibrational ball milling. Solid-state characterization with X-ray powder diffraction (XRPD) and differential scanning calorimetry (DSC) revealed that the various blends could be prepared as homogeneous, single phase co-amorphous formulations indicated by the appearance of an amorphous halo in the XRPD diffractograms and a single glass transition temperature (Tg) in the DSC measurements. In addition, the Tgs of the co-amorphous mixtures were significantly increased over those of the individual drugs. The drugs remained chemically stable during the milling process and the co-amorphous formulations were generally physically stable over at least 6 months at 40 °C under dry conditions. The dissolution rate of all co-amorphous drug-amino acid mixtures was significantly increased over that of the respective crystalline and amorphous pure drugs. Amino acids thus appear as promising excipients to solve challenges connected with the stability and dissolution of amorphous drugs.